Phenol and methyl ethyl ketone are important products in the chemical industry. For example, phenol is useful in the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, alkyl phenols, and plasticizers, whereas methyl ethyl ketone can be used as a lacquer, a solvent and for dewaxing of lubricating oils.
The most common route for the production of methyl ethyl ketone is by dehydrogenation of sec-butyl alcohol (SBA), with the alcohol being produced by the acid-catalyzed hydration of butenes. For example, commercial scale SBA manufacture by reaction of butylene with sulfuric acid has been accomplished for many years via gas/liquid extraction.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene relative to that for butenes is likely to increase, due to a developing shortage of propylene. Thus, a process that uses butenes instead of propylene as feed and co-produces methyl ethyl ketone rather than acetone may be an attractive alternative route to the production of phenol.
It is known that phenol and methyl ethyl ketone can be co-produced by a variation of the Hock process in which sec-butylbenzene is oxidized to obtain sec-butylbenzene hydroperoxide and the peroxide decomposed to the desired phenol and methyl ethyl ketone. An overview of such a process is described in pages 113-241 and 261-263 of Process Economics Report No. 22B entitled “Phenol”, published by the Stanford Research Institute in December 1977.
In addition, it is known that sec-butylbenzene can be produced by alkylating benzene with n-butenes over an acid catalyst. For example, in our International Patent Publication No. WO06/015826, we have described a process for producing phenol and methyl ethyl ketone, in which benzene is initially contacted with a C4 alkylating agent under alkylation conditions with a catalyst comprising zeolite beta or a molecular sieve of the MCM-22 family to produce an alkylation effluent comprising sec-butylbenzene. The sec-butylbenzene is then oxidized to produce a hydroperoxide and the hydroperoxide is decomposed to produce phenol and methyl ethyl ketone. The oxidation step can be conducted with or without a catalyst under conditions including a temperature between about 70° C. and about 200° C., such as about 90° C. to about 130° C., and a pressure of about 0.5 to about 10 atmospheres (50 to 1000 kPa). Suitable catalysts are said to include the N-hydroxy substituted cyclic imides described in Published U.S. Patent Application No. 2003/0083527.
Although the chemistry involved in the alkylation of benzene with butenes is very similar to ethylbenzene and cumene production, as the carbon number of the alkylating agent increases, the number of product isomers also increases. For example, ethylbenzene has one isomer, propylbenzene has two isomers (cumene and n-propylbenzene), and butylbenzene has four isomers (n-, iso-, sec-, and t-butylbenzene). For sec-butylbenzene production, it is important to minimize n-, iso-, t-butylbenzene, and phenylbutenes by-product formation since these by-products have boiling points very close to sec-butylbenzene and hence are difficult to separate from sec-butylbenzene by distillation (see table below).
ButylbenzeneBoiling Point, ° C.t-Butylbenzene169i-Butylbenzene 171s-Butylbenzene 173n-Butylbenzene 183
By-product formation can of course be minimized by using a pure n-butene feed, but in practice it is desirable to employ more economical butene feeds, such as Raffinate-2, to produce sec-butylbenzene. A typical Raffinate-2 contains 0-1% butadiene and up to 5% isobutene. With this increased isobutene level in the feed, a higher by-product make is expected even with a highly selective alkylation catalyst.
Moreover, iso-butylbenzene and especially tert-butylbenzene are known to be inhibitors to the oxidation of sec-butylbenzene to the corresponding hydroperoxide, a necessary next step for the production of methyl ethyl ketone and phenol. Even in the presence of catalysts such as cyclic imides, these impurities are found to decrease the selectivity to sec-butylbenzene hydroperoxide while themselves being oxidized at a much slower rate than sec-butylbenzene. Thus, since oxidation is generally operated at low conversion rates with recycle of unreacted sec-butylbenzene, impurities such as iso-butylbenzene and tert-butylbenzene are likely to build up in the recycle stream.
Thus any successful commercial process for producing phenol via alkylation of benzene to sec-butylbenzene is likely to require provision for reducing at least the level of tert-butylbenzene and preferably the level of iso-butylbenzene in the alkylation effluent.
Thermodynamic calculations suggest that iso-butylbenzene and particularly tert-butylbenzene can be dealkylated at a faster rate than sec-butylbenzene. Thus, according to the present invention, a process is provided for reducing the level of tert-butylbenzene and, if present, iso-butylbenzene in a mixed butylbenzene feed, such as that generated in the alkylation of benzene with Raffinate-2, by subjecting the feed to catalytic dealkylation. The dealkylation is found to selectively convert the tert-butylbenzene and iso-butylbenzene in the feed to benzene and iso-butene, with the latter reacting with the butylbenzenes in the feed to produce dibutylbenzenes or, in the presence of hydrogen, being reduced to iso-butane. The benzene and dibutylbenzenes or iso-butane can readily be removed from the dealkylation effluent by distillation and the unconverted C10 stream, which is rich in sec-butylbenzene as compared with the feed, can then be fed to the oxidation step.